Due to self-emission and no dependency on viewing angle, as well as capability of plane emission and reduction in thickness, application of an organic EL device to a display of a TV or a mobile phone, an illumination or the like has been studied.
An organic EL device has a configuration in which a thin film of an organic compound is sandwiched between a cathode and an anode. The methods for forming a thin film are roughly divided into the deposition method and the coating method. The deposition method is a method in which a low-molecular weight compound is mainly used, and a thin film is formed on the substrate in vacuum. The industrial-base-production thereof proceeds. On the other hand, the coating method is a technique in which a thin film is formed on the substrate by using a solution (e.g. inkjet and printing). The deposition method has a high material utilization efficiency, and hence is suited to an increase in area and an enhancement of resolution. This method is a technology which is deemed to be indispensable in an organic EL display in the future.
The vacuum deposition method using a low-molecular material has an extremely low utilization efficiency of the material. In addition, if the size is increased, the degree of distortion of a shadow mask becomes large. Therefore, deposition on a large-sized substrate is difficult, and as a result, fabrication of a display using a large-sized organic EL substrate is difficult. In addition, the production cost also increases.
On the other hand, in the case of a high-molecular weight material, it is possible to form a homogenous film by applying a solution obtained by dissolving the high-molecular weight material in an organic solvent. A coating method represented by the inkjet method or the printing method can be used by utilizing such a nature of a high-molecular weight material. Therefore, it is possible to enhance the material utilization efficiency to near to 100%. In addition, since it can be applied to substrates of various shapes ranging from a small-sized substrate to a large-sized substrate, the production cost of the device can be significantly reduced.
However, in general, the coating method is not suited to a stacked-type device, and has a problem that it is not easy to improve the performance of the device.
The reason for unsuitability of applying the coating method to a stacked-type device is that, when stacking layers, films which have been formed in advance are inevitably dissolved. Specifically, in a stacked-type organic EL device fabricated by the coating method, a hole-injecting layer and a hole-transporting layer have to be insoluble in a solvent used when an emitting layer is formed.
Therefore, at present, most of organic EL devices obtained by the coating method are limited to a two-layer structure in which a hole-injecting layer is formed by a water-dispersion liquid of polythiophene:polystyrene sulfonic acid (PEDOT:PSS), and an emitting layer is formed by using an aromatic organic solvent such as toluene. Since the PEDOT:PSS layer is not dissolved in toluene, it is possible to fabricate such two-layer structure.
Accordingly, this method cannot be applied to a case where a hole-transporting layer is formed on a hole-injecting layer, and an emitting layer is further formed.
As represented by PEDOT/PSS, in many cases, a water-soluble or an alcohol-soluble material is used in a hole-injecting layer. Therefore, a hole-transporting material is formed into a hole-transporting layer and stacking on a hole-injecting layer by using an organic solvent other than a water-based or an alcohol-based organic solvent (e.g. toluene, xylene, dioxane and cyclohexanone).
Here, as the hole-transporting material, use of a heat- or light-cross-linking polymer is proposed (see Patent Document No. 1). However, a light-cross linking material has a problem that it deteriorates by exposure to light, and its hole-transporting property, luminous efficiency and device life are lowered. On the other hand, a heat-cross-linking material has a problem that it shrinks or suffers pinhole generation after the heat-cross-linking.
Further, if shrinkage does not occur after the heat-cross linking, it may be dissolved in a film-forming solvent used for stacking an emitting layer (Patent Document Nos. 2 to 4).
In addition, there is a technology in which a material obtained by bonding a polymerizable substituent to a low-molecular compound is used, and the material is applied, followed by polymerization to lower the solubility in a coating solvent for other layers, whereby a multi-layer structure is formed (see Patent Document 5). However, a low-molecular compound has a tendency that it is likely to be crystallized. Therefore, it has a problem that a high-quality thin film is unlikely to be formed.
As a hole-transporting material which can be used in a coating type multilayer organic EL device, a polymer in which a cross-linking group is introduced into the main chain is disclosed. However, as for these materials, if a cross-linking group unit is included, it becomes a ternary, or a quaternary (in the greatest case) compound, and as a result, it has a significantly complicated structure. Therefore, performance reproducibility may be difficult (Patent Document Nos. 6, 7 and 8). In addition, a material in which a cross-linking group is introduced into its terminal is disclosed. However, due to copolymerization of the same carbazole group as that of a soluble unit, minor adjustment of film-forming properties, adhesiveness, and further, electronic properties or the like by introducing a plurality of substituents becomes difficult. Further, since this material is an oligomer material having a molecular weight of about 2000 to 3000, adverse effects exerted by unreacted cross-linking groups on the lowering of device properties are concerned (Patent Document 9).